U.S. Pat. No. 3,767,756 describes a process for spinning para-aramids to provide filaments having excellent as-spun tenacity, modulus and breaking elongation. This patent also discloses heat treatment of the as-spun fibers, preferably in an inert atmosphere, to provide filaments having a much higher modulus but lower breaking elongation. The tenacity usually decreases slightly from its already high value.
Of the para-aramids, poly(p-phenylene terephthalamide) (PPD-T) has achieved the greatest commercial success. U.S. Pat. No. 3,869,430 describes a heat treatment process for as-spun PPD-T filaments such as those prepared according to the process of U.S. Pat. No. 3,767,756 whereby the filaments are heated in a zone at a temperature of at least 150.degree. C., preferably in an inert atmosphere, under a tension of at least 0.5 gpd (0.45 g/dtex) but less than the tension required to draw the filaments more than 1.03 times their initial length. The preferred temperature of the heating zone is 250.degree.-600.degree. C., most preferably 450.degree.-580.degree. C.
Although the prior art para-aramid heat treating processes teach that the heat treatment may be in an inert gas, or superheated steam or by a hot plate or infrared rays, hot pins, hot slots, hot rolls or liquid heating baths, only inert gas heat treatment has been demonstrated. However, hot gas treatment processes require either very long tubes for treatment or slow speed processing.
Since para-aramid filaments may be spun at spinning speeds greater than 457 m/min, it has not been practical to provide a coupled para-aramid spinning and heat treating process because even a one second heat treatment at 457 m/min requires a tube 7.6 m long.
This invention provides a process for heat treating para-aramid filaments at high speed under tension comprising the steps of (1) feeding para-aramid filaments having a filament tenacity of at least 18 gpd and a breaking elongation of at least 3.5% under a tension of 0.6 to 4.0 gpd to a first array of closely spaced heated rolls having a surface temperature of 175.degree.-300.degree. C., (2) passing the filaments over the rolls to provide at least 180.degree. arc of contact with each roll, (3) stretching the filaments 0 to 1.6%, (4) passing the filaments over a second array of closely spaced heated rolls having a surface temperature of 175.degree.-300.degree. C. to provide at least 180.degree. arc of contact with each roll, (5) cooling the filaments and collecting the filaments in a suitable package. Preferably the filaments are stretched 0.2 to 1.0% between the first and second arrays of heated rolls. Preferably the filaments are cooled by passing them over an array of cooled rolls to provide at least 180.degree. arc of contact with each roll while maintaining 0.6 to 4.0 gpd tension on the entering filaments. Preferably the rolls are arranged in a staggered array to minimize machine size. The number of rolls in each array and their diameter must allow sufficient contact with the filaments that the required filament tension can be achieved. The total number of rolls must provide sufficient contact with the filaments to heat them to the desired temperature at the required process speed. Most preferably the first array of heated rolls consists of 5 induction heated rolls having a diameter of no more than 38 cm and the second array of heated rolls consists of 7 induction heated rolls having a diameter of no more than 38 cm. Most preferably the filaments are cooled by passing them back and forth in serpentine fashion over 8 cooled rolls having a diameter of no more than 30.5 cm to provide at least 180.degree. contact with each roll while maintaining a tension of 0.6 to 4.0 gpd on the entering filaments. Most preferably the process is coupled directly with the spinning process.
This invention also provides a compact apparatus for heat treating a moving threadline comprising a first array of closely spaced heated rolls capable of operating at a surface speed in excess of 274 m/min and surface temperatures of 175.degree.-300.degree. C. followed by a second array of closely spaced heated rolls capable of operating at a surface speed in excess of the surface speed of the first array of rolls and having a surface temperature of 175.degree.-300.degree. C. and a third staggered array of closely spaced cooling rolls capable of operating at a surface speed that will maintain 0.6 to 4.0 gpd tension on yarn fed to the cooling rolls and followed by a device for collecting the filaments. Preferably the first array consists of 5 rolls having a diameter of no more than 38 cm, the second array consists of 7 rolls having a diameter of no more than 38 cm and the third array consists of 8 rolls having a diameter of no more than 30 cm.